Histoplasma capsulatum is a dimorphic, pathogenic fungus that produces a wide spectrum of illness ranging from a mild, acute pulmonary infection to a life- threatening progressive, disseminated disease. Successful resolution of infection requires an interaction between T cells and macrophages. Although the latter are the principal habitat and primary effector cells, T cells play a vital and essential role in host resistance to this fungus. We have demonstrated that, in mice, in vivo depletion of either CD4+ or CD8+ T cells is associated with increased severity of infection. Elimination of CD4+ T cells results in death of mice challenged with a sublethal inoculum. Depletion of CD8+ T cells impairs the efficiency of clearance of H. capsulatum from viscera. Thus, optimal clearance of infection requires both T cell subsets. Prior studies with intracellular non-viral pathogens suggest that cytotoxic CD4+ or CD8+ play an important role in recovery from infection. In this proposal, we will test the hypothesis that cytotoxic CD4+ and CD8+ T cells contribute to host defenses against this fungus. The specific objectives of this grant are to: l) Examine the in vitro immunobiological activity of murine H. capsulatum-reactive cytotoxic CD4+ and CD8+ T cells. Specifically, T cell clones will be generated and tested for their capacity to lyse yeast- infected macrophages. Moreover, we will determine the fate of H. capsulatum yeasts in macrophages exposed to cytotoxic T cells, and map the antigens recognized by cytotoxic T cells; 2) Analyze the in vivo functional activity of murine cytotoxic T cell clones. T cell clones will be tested for their capacity to confer protection against a challenge with H. capsulatum, and 3) Determine if human H. capsulatum-reactive CD4+ and CD8+ T cells express cytotoxic activity. Human T cell clones will be analyzed for their ability to lyse yeast-infected human macrophages. Subsequently, we will determine if human cytotoxic T cells alter the fate of yeast cells in human macrophages. These studies will provide further insight into the mechanisms of T cell-mediated immunity.